The present invention relates to a vehicle control system and a corresponding method for determining a driver's desired yaw rate of a vehicle.
When operating a vehicle there is a risk that the driver loses control of the vehicle's motion in the yaw plane, typically due to insufficient road friction resulting in that the vehicle may understeer or oversteer. To assist the driver in keeping control of the vehicle's yaw motion, modern vehicles are equipped with different vehicle movement control systems such as Electronic Stability Control (ESC) systems. Generally, ESC systems detect yaw instabilities by comparing an actual yaw motion in the yaw plane of the vehicle with a desired yaw rate. Subsequently, the ESC system reduces any differences between actual and desired yaw motions by applying brake pressures in a controlled manner at the individual wheels of the vehicle.
The desired yaw rate is generally calculated based on the front wheel steering angle, vehicle speed and wheel base, or combinations thereof. More specifically the driver is generally expected to direct the front wheels by means of the steering wheel in the direction which he wants the vehicle to rotate, according to some models of vehicle movement such as for example some versions of the recognized linear bicycle model:
            ω      bicycle        ⁡          (              v        ,                  δ          FW                    )        =            v      ⁢                          ⁢              δ        FW              b  
where v is the vehicle speed, b is the constant wheel base of the vehicle, and δFW is the front wheel steering angle. For example, if the vehicle oversteers in a left hand curve, i.e. the left hand yaw rate of the vehicle exceeds the curvature of the left hand curve, a driver is expected to rotate the steering wheel to the right into a steering angle compensating the oversteering.
A drawback with existing ESC-systems is that the desired yaw rate is not always estimated satisfyingly well, especially when the vehicle is in more severe situations of yaw instability.
Thus, there is a need of for example a method for estimating a desired yaw rate of a vehicle which works well in severe situations of yaw instability.
It is desirable to provide a method for estimating a desired yaw rate of a vehicle which works well in severe situations of yaw instability.
According to a first aspect of the invention a method is provided for calculating a driver's desired yaw rate of a vehicle for use in a vehicle movement control system, comprising the steps of determining the current yaw rate of the vehicle and determining the rate of the vehicle's steering wheel rotation. The method further comprises the steps of calculating a first desired yaw rate of the vehicle based on said determined current yaw rate of the vehicle and said determined rate of the vehicle's steering wheel rotation, the desired yaw rate being further calculated based on the assumption that the driver applies a rate of steering wheel rotation as function of the driver's perceived error in yaw rate, and finally the step of providing said first desired yaw rate as an input to said vehicle movement control system for controlling the vehicle.
In the context of this application, the “rate of the vehicle's steering wheel rotation” is defined as the rotational speed of the steering wheel. It is not equivalent with an actual angle of the steering wheel, but rather a change of the steering wheel angle per time unit. Further, in the context of this application “yaw rate” should be understood as being equivalent to lateral acceleration, curvature or turning radius etc., as these measurements all indicate corresponding vehicle movements E.g. the yaw rate may be the angular velocity of the vehicle rotation in the horizontal plane. When “desired yaw rate” is used it means the yaw rate (or equivalent) which the driver desires to achieve. For example, if the driver is holding the steering wheel still, the driver is assumed to be satisfied with the yaw rate of the vehicle. Further, in the context of this application, a “vehicle movement control system” means any control system for controlling the vehicle's movements e.g. an Electronic Stability Control (ESC) System, a steer-by-wire control system, an active steering arrangement or a suspension control system etc.
The inventor has realized that during severe skidding, drivers tend to abandon the type of steering wheel angle-focused steering control assumed by conventional control systems (e.g. ESC systems), for example implementing the linear bicycle model as discussed above, and instead adopt a type of steering control where the exact steering wheel angle is not controlled, but rather the rate of steering wheel angle. Therefore, a conventional ESC system would misinterpret the driver's intentions completely. However, by providing a method as defined above calculating the desired yaw rate based on the rate of steering wheel rotation and the vehicles actual yaw rate, a safer control system may be provided for a vehicle which works well when the vehicle is in severe situations of yaw instability, such as during severe skidding. The method further allows for a much faster control of the vehicle by means of the movement control system. During tests conducted by the inventor, obtaining a rate of steering wheel rotation only requires about one third of the time of what is required for obtaining a steering wheel angle value. Specifically, the average result in tests show that a confident value may be obtained in about 0.2 s for rate of rotation-values compared to about 0.6 s for angle-values. The effect of this is that a much safer control system may be provided, e.g. when the control system is a ESC System the wheels of the vehicle may be individually braked at an earlier point in time, to reduce the risk of an accident.
Moreover, the method may interpret the driver's applied rate of steering wheel rotation as the more wrong the vehicle's rotation feels to the driver, the faster the driver will spin the steering wheel to counter the skidding.
In one embodiment the desired yaw rate may be defined by the equation
            ω      *        =          ω      +              k        ⁢                              ⅆ                          δ              SW                                            ⅆ            t                                ,where ω* is the desired yaw rate, ω is the actual yaw rate, δSW is the steering wheel angle and k a the scaling parameter.
In one embodiment an equation for representing the driver behavior may be
            ⅆ              δ        SW                    ⅆ      t        =                    (                              ω            *                    -          ω                )            k        =                  ω        error            k      where ωerror=ω*−ω is the deviation between desired and actual yaw rate.
In one embodiment the function describing the relation between how the driver applies a rate of steering wheel rotation as a function of the driver's perceived error in yaw rate is a linear scaling function. In another embodiment, the function is a non-linear scaling function, such as an exponential function.
According to another embodiment of the present inventive concept, the method further comprises the step of calculating a severity of the yaw state of the vehicle.
In the context of this application the severity of the yaw state should be understood as the amount of skidding the vehicle is in and/or the driver's rate of the steering wheel rotation. The severity may for example be estimated in terms of the absolute rate of steering wheel rotation (S=|dδSW/dt|), the absolute deviation between actual yaw rate and desired yaw rate according to a conventional vehicle dynamics-based estimate such as the linear bicycle model, or low-pass filtered versions of either of these quantities in order to reduce the frequency of transitions between desired yaw rate estimates. A low-pass filter in this sense is not necessarily a filter only letting through a certain span of frequencies, but rather a general filter using hysteresis or time span operations, where the primary object of the filter being to reduce the frequency of transitions between the desired yaw rate estimations to obtain a more stable value of the desired yaw rate estimation.
According to yet another embodiment of the present inventive concept, the step of calculating a severity of the yaw state of the vehicle is a calculation based on the absolute rate of the steering wheel rotation.
According to another embodiment of the present inventive concept, the step of calculating a severity of the yaw state of the vehicle is a calculation based on a deviation between actual yaw rate and desired yaw rate being calculated according to a conventional vehicle dynamics-based estimate such as the linear bicycle model.
According to yet another embodiment of the present inventive concept, if said severity is below a first threshold value the method further comprises the steps of detecting the steering wheel angle and calculating a second desired yaw rate based on the detected steering wheel angle, and providing the second desired yaw rate instead of the first desired yaw rate as an input to the vehicle movement control system for controlling the vehicle.
Thereby, the method may identify the amount of skidding (i.e. understeering/oversteering) and/or absolute rate of the steering wheel rotation of the vehicle, and if the vehicle's severity of the skidding/steering wheel rotation rate is in a state below the threshold value, e.g. when no or very little skidding is occurring, a second desired yaw rate calculated according to a conventional vehicle dynamics-based estimate such as the linear bicycle model may be used instead of the first desired yaw rate.
Thereby, the desired yaw rate may be calculated in different ways depending on the severity of the yaw/skidding state of the vehicle.
According to yet another embodiment of the present inventive concept, if said severity is between a first threshold value and a second threshold value the method further comprises the steps of detecting the steering wheel angle and calculating a second desired yaw rate based on said detected steering wheel angle, and further calculating a third desired yaw rate based on the first desired yaw rate and the second desired yaw rate, and thereafter providing said third desired yaw rate instead of the first desired yaw rate as an input to said vehicle movement control system for controlling the vehicle.
Thereby, the method may identify the amount of skidding (i.e. understeering/oversteering) and/or absolute rate of the steering wheel rotation of the vehicle, and if the vehicle's severity of the skidding/yaw rate is in a state above the first threshold value but below a second threshold value, e.g. when some skidding is occurring but not more than what is defined by the second threshold value, a third desired yaw rate calculated as a combination of the first and second desired yaw rates may be used for controlling the vehicle by means of the vehicle movement control system. Thereby, the desired yaw rate may be calculated in different ways depending on the severity of the skidding/yaw state of the vehicle.
According to another embodiment of the present inventive concept, the step of calculating the third desired yaw rate comprises a weighting function where more weight is given to the first desired yaw rate in higher severity levels, and opposite, more weight is given to the second desired yaw rate in lower severity levels.
Thereby, a traditional steering wheel angle-focused steering control such as the linear bicycle model may be used during normal driving when the vehicle is not in a skidding state, and when the vehicle is in a severe skidding/yaw state, the desired yaw rate may be calculated based on the rate of steering wheel rotation and the vehicles actual yaw rate. Thus a safer control system may be provided for a vehicle which works well both when the vehicle is in a normal state and when the vehicle is in severe situations of yaw instability, such as severe skidding.
According to yet another embodiment of the present inventive concept, a parameter is used for scaling the rate of steering wheel rotation when calculating the desired yaw rate. Thereby, the rate of steering wheel rotation may be scaled to be adapted to a specific vehicle steering arrangement.
According to another embodiment of the present inventive concept, the parameter is a constant system parameter. Thereby, the first desired yaw rate by means of the scaling factor may be adapted to the vehicle's system for providing a relevant desired yaw rate.
According to yet another embodiment of the present inventive concept, the parameter may be dynamically adapted to the vehicle's driver by observing the driver's behavior of steering during normal driving. Thereby, the first desired yaw rate may be adapted to the drive style of the driver resulting in a more accurate estimation of the calculated desired yaw rate. E.g. a driver with higher aggressiveness when handling the steering wheel will need to turn the steering wheel faster for achieving the same desired yaw rate as a driver having a less aggressive driving style.
According to yet another embodiment of the present inventive concept, the vehicle movement control system is an Electronic Stability Control (ESC) system. Thereby, the ESC system may work well even during severe skidding. Thus, by calculating the desired yaw rate in a more accurate manner, the braking of each individual wheel of the vehicle by means of the ESC System may be improved, and thereby providing a safer vehicle for the driver.
According to another embodiment of the present inventive concept, the vehicle movement control system is a control system for a steer-by-wire system. Thereby, analogously with the benefit as described above, by calculating the desired yaw rate in a more accurate manner, the steer-by-wire may achieve a safer vehicle for the driver.
According to another aspect of the invention there is provided a vehicle control system for calculating a driver's desired yaw rate of a vehicle for use in a vehicle movement control system, comprising determining means for determining the current yaw rate of the vehicle, determining means for determining the rate of the vehicle's steering wheel rotation, and calculating means for calculating a first desired yaw rate of the vehicle based on said determined current yaw rate of the vehicle and said determined rate of the vehicle's steering wheel rotation, the desired yaw rate being further calculated based on the assumption that the driver applies a rate of steering wheel rotation as function of the driver's perceived error in yaw rate, and providing means for providing said first desired yaw rate as an input to said vehicle movement control system for controlling the vehicle.
The advantages of the vehicle control system as defined above are largely analogous to the advantages of the method as described above. That is, a safer control system may be provided for a vehicle which works well when the vehicle is in severe situations of yaw instability, such as during severe skidding.
According to still another aspect of the invention there is provided a computer-readable storage medium storing a program which causes a computer to execute a control method according to any of the embodiments as described above.
According to a still further aspect of the invention there is provided a computer program product comprising a computer readable medium having stored thereon computer program means for calculating a driver's desired yaw rate of a vehicle for use in a vehicle movement control system, wherein the computer program product comprises code for determining the current yaw rate of the vehicle, code for determining the rate of the vehicle's steering wheel rotation, code for calculating a first desired yaw rate of the vehicle based on said determined current yaw rate of the vehicle and said determined rate of the vehicle's steering wheel rotation, the desired yaw rate being further calculated based on the assumption that the driver applies a rate of steering wheel rotation as function of the driver's perceived error in yaw rate, and code for providing said first desired yaw rate as an input to said vehicle movement control system for controlling the vehicle.
The advantages of the computer program product as defined above are largely analogous to the advantages of the method as described above. That is, a safer control system may be provided for a vehicle which works well when the vehicle is in severe situations of yaw instability, such as during severe skidding.
Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled addressee realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.